Earthquakes are caused by the resultant relative slippage of the earth crust, generally along or near major tectonic plate boundaries. In certain parts of the world, continuous differential movement occurs between one section of the earth's crust and an adjacent one, causing an accumulation of strain at the boundary. When the stresses caused by this strain accumulation exceed the strength of the earth's materials, a slip occurs between two portions of the earth's crust and tremendous amounts of energy are released. This energy propagates outward from the focus or origin of the earthquake in the form of body and surface elastic stress waves.
The energy released during an earthquake event is transmitted through the earth's crust in the form of body and surface seismic waves. The body waves are composed of P-(compression) waves and S-(shear) waves, with the P-wave traveling significantly faster than the S-wave. The surface waves of most interest are the Rayleigh wave and the Love wave. The Love wave travels faster than the Rayleigh wave. The total energy transported is represented almost entirely by the Rayleigh, the S- and the P-waves, with the Rayleigh wave carrying the largest amount of energy, the S-wave an intermediate amount, and the P-wave the least. The velocity of the P-wave is almost double that of the S-wave, and the velocity of the S-wave is only slightly greater than the Rayleigh wave.
At some distance from an earthquake disturbance a particle at the earth's surface first experiences a displacement in the form of an oscillation at the arrival of the P-wave followed by a relatively quiet period leading up to another oscillation at the arrival of the S- and Rayleigh waves. These events are referred to as the minor tremor and the major tremor at the time of arrival of the Rayleigh wave. The body and surface waves are monitored during an earthquake to gauge the earthquake's intensity.
Earth ground motions experienced during an earthquake are actually quite complex due to the variation in the earth's crust, from strong stiff bedrock to soft weak soils. Considerable energy can be transmitted through the bedrock, and it appears that in many cases the main forces acting on soil elements in the field during earthquakes are those resulting from the upward migration of shear motions from the underlying rock formations. Although the actual wave pattern may be very complex, the resulting ground motion imposed on the soil and the overlying structure are predominantly from the upward propagation of the S-wave components from the underlying bedrock. Deposits of thick soft soils can give rise to amplification of these ground motions in particular in the long period (low frequency) content of the earthquake induced shaking. Such amplification of earthquake induced ground motions can cause extensive damage to buildings, bridges, pipelines, embankments, dams, slopes, and other structures and works constructed on soft soil deposits.
The factors that effect the amplification of earthquake induced ground motions are soil type, grain size distribution, compactness of the soil, thickness of the soil deposit, depth to groundwater, and the magnitude and number of the strain reversals. Deposits of soft soils, such as silts and clays are most likely to amplify ground motions during an earthquake. Structures constructed on such soils can be extensively damaged by even a moderate size earthquake. Two recent earthquakes, the 1985 Michoacan (Mexico) and the 1989 Loma Prieta (Calif.), highlight the extensive earthquake induced damage to structures located on soft soil deposits. The 1985 Michoacan earthquake caused only moderate damage in the vicinity of its epicenter but caused extensive damage to structures located on a thick deposit of soft silts and clay some 350 km away in Mexico City. Likewise, the 1989 Loma Prieta earthquake caused minor damage in the vicinity of its epicenter but caused moderate to extensive damage to structures located on the San Francisco Bay mud some 100 km away.
Conventional seismic isolation systems to minimize or prevent damage to a structure by isolating the structure from ground motions during an earthquake consist of the following:
1) sliding bearings with energy absorbing properties to isolate the structure from horizontal earthquake induced ground motions, such as lead rubber, steel neoprene/rubber and fiber reinforced elastomer,
2) sliding bearings with fluid dampers to both isolate the structure from earthquake induced ground motion and modify the structural response to minimize damage,
3) passive mass damping systems consisting of a pendulum suspended weight and associated dampers to absorb vibratory energy and minimize damage to the structure,
4) active mass damping systems consisting of a sensor and computer controlled movement of a mass to minimize vibration and damage to the structure,
5) pneumatic or fluidized foundation isolation system to reduce earthquake induced ground motions being transmitted to the structure.
The above methods have had mixed success in minimizing damage and vibrations to a structure during an earthquake. The passive and active mass damping systems have been shown to be successful during strong winds and minor earthquakes. Bearing isolation systems have in some circumstances, e.g. the 1994 Northridge (Calif.) earthquake, demonstrated to provide poor if any base isolation of the structure from the earthquake induced ground motion. The mass damping systems have demonstrated some protection of a structure due to earthquake vibrations; however, they are expensive, and difficult to implement in existing structures. The energy absorbing sliding bearing systems can be implemented in existing structures; however, their performance during actual earthquake events appear limited in isolating the structure from earthquake induced ground motions and minimizing structural damage.
The main forces acting on soil elements in the field during earthquakes are those resulting from the upward migration of shear motions from the underlying rock formations. Although the actual wave pattern may be very complex, the resulting repeated and reversing shearing deformations, imposed on the soil by the S-wave components are the principal cause of a phenomenon known as liquefaction, which occurs in saturated fine sand, silty sand and silt deposits. When these soil deposits are subjected to repeated shear strain reversals, the volume of the soil decreases with each cycle, i.e. the soil contracts, and due to the lack of drainage of these saturated soils, the soil pore water pressure rises. As the soil pore water pressure rises, the grain to grain contact pressure becomes smaller, until eventually the grain to grain contact pressure drops to zero and the soil loses all of its shear strength and acts like a fluid. Liquefaction can occur in loose saturated fine sands, silty sands and silts as a result of earthquakes, blasting or other shocks.
The factors that effect the occurrence of liquefaction are soil type, grain size distribution, compactness of the soil, soil permeability, and the magnitude and number of the strain reversals. Fine cohesionless soils, fine sand or fine cohesionless soils containing moderate amounts of silt are most susceptible to liquefaction. Uniformly graded soils are more susceptible to liquefaction than well graded soils, and fine sands tend to liquefy more easily than coarse sands or gravelly soils. Moderate amounts of silt appear to increase the liquefaction susceptibility of fine sands; however, fine sands with large amounts of silt are less susceptible, although liquefaction is still possible. Recent evidence indicates that sands containing moderate amounts of clay may also be liquefiable.
Current methods for evaluating the liquefaction potential of soils consist of two basic approaches, laboratory tests and in situ tests. The laboratory methods require undisturbed soil samples which are difficult to impossible to obtain. The laboratory test methods involve cyclic triaxial, cyclic direct shear and cyclic torsional triaxial tests. All of these tests apply a cyclic shear stress reversal upon the soil specimen. At the present time, there is not a method for obtaining undisturbed samples, in which the in situ stress state, void ratio or structure have been preserved in cohesionless soils. Therefore, laboratory methods are considered only qualitative tests in assessing the potential of a soil to liquefy. The in situ methods currently consist of five (5) types, with four (4) of the methods; 1) the Standard Penetration Test (SPT); 2) the Cone Penetration Test (CPT); 3) the Piezocone Penetration Test (PCPT) and 4) the Seismic Waves Test (SWT) being indirect empirical methods and the fifth method an in situ cyclic stress reversal test being a direct in situ measurement of a soil's tendency to liquefy. The present in situ methods are capable of determining whether a particular soil horizon has the potential to liquefy and under what earthquake ground motions it will most likely liquefy.
Since shear waves can not propagate through a fluid, a liquefied soil horizon will act as a seismic isolation barrier and stop/inhibit the upward propagation of earthquake induced shear wave ground motions to overlying soils and structures. To avoid liquefaction related damage to the surface the liquefied soil horizon needs to be at a depth greater than 5 times its liquefiable thickness, Ishihara (1985) and Youd & Garris (1995).
Electro-osmosis involves the application of a direct current (dc) between electrodes inserted in the saturated soil, that gives rise to pore fluid movement from the source electrodes towards the sink electrodes and thus modifies the soil pore water pressures. Electro-osmosis has been used in applications such as 1) improving stability of excavations, 2) decreasing pile driving resistance, 3) increasing pile strength, 4) stabilization of soils by consolidation or grouting, 5) dewatering of sludges, 6) groundwater lowering and barrier systems, 7) increasing petroleum production, 8) removing contaminants from soils, and 9) preventing soil liquefaction during an earthquake event. Electro-osmosis uses a dc electrical potential difference applied across the saturated soil mass by electrodes placed in an open or closed flow arrangement. The dc potential difference sets up a dc current flowing from the source electrodes to the sink electrodes. In most soils the soil particles have a negative charge. For those negatively charged soils, the source electrodes is the anode electrode, the sink electrode is the cathode electrode, and ground water migrates from the anode electrode towards the cathode electrode. In other soils, such as calcareous soils (e.g. limestone), the soil particles carry a positive charge. In those positively charged soils, the source electrode is the cathode electrode, the sink electrode is the anode electrode, and ground water migrates from the cathode electrode towards the anode electrode.
An “open” flow arrangement at the electrodes allows an ingress or egress of the pore fluid. Due to the electrically induced transport of pore water fluid, the soil pore water pressures are modified to enable excavations to be stabilized or pile driving resistance to be lowered. Electro-osmosis is not used extensively due to the high cost of maintaining the dc potential over long periods of time and the drying out and chemical reactions that occur if the system is activated for long periods of time.
Monitoring ground motion and activating safety devices or active mass damping systems prior to the arrival of a major earthquake can in some cases reduce damage. Such a forecasting system can be used to close gas valves or cutoff electricity to the effected area. Such systems may include a tuned pendulum system, that upon the onset of certain ground motion magnitude and frequency, the pendulum motion sets off an alarm, activates a switch or closes a gas valve prior to the arrival of the major tremor of the earthquake. Alternatively, a heavy sliding or rotating mass can be used to activate a similar switch, contact or value, by sizing the mass that upon experiencing certain ground motions the mass slides or rotates and activates a switch, contact or closes a valve prior to the arrival of the major destructive earthquake tremor.